Optical fiber is a self-evident choice as the transmission medium for a trunk network, because trunk connections usually need a high transmission capacity; the transmission distances used are long, and existing routes are often found for the cables. Even for subscriber connections (the line between the local exchange and the subscriber) the situation is rapidly changing, because various services implemented with multimedia and demanding a high transmission rate will soon be everyday services also from the viewpoint of the private consumer. Changing the subscriber network into an optical network is a technical-economical question involving a conflict between expenses, on the one hand, and the advantages achieved on the other. Purely from the technical point of view, it is obvious that the capacity of a metal pair cable is considerably less than the capacity of an optical fiber. This is obvious although nowadays efficient studies have begun to examine the possibility of using the ordinary subscriber line (metal pair cable) for high-speed data transmission. Such solutions are primarily the new HDSL, ADSL and VDSL technologies which offer new possibilities for transferring data and video via the pair cable of the telephone network to subscriber terminals.
The international standardization organization ETSI (European Telecommunications Standards Institute) has defined the TCM technique as one of the transmission techniques to be used for narrow-band interactive services in an optic access network (the term access network refers to the part of the data transmission network which enables the users to access the services provided by the network). Current TCM systems operate so that the transceiver units alternate in transmitting bursts in the fiber or line. This principle is illustrated in FIGS. 1 and 2, which describe the connection at two different moments in time. The transceivers A and B send and receive alternatively; as one transmits, the other receives, and vice versa. Because of this the technique is referred to as "ping-pong". Therefore, there is only one burst 12 proceeding in the fiber or line 11.
FIG. 3 illustrates one operation cycle of this kind of system, the cycle being referred to with the reference symbol Ta. The length of the operational cycle is 2(Tp.times.Lmax)+Tg1+Tg2+2T1, where Tp is the propagation velocity in the fiber (.apprxeq.4.85 .mu.s/km), Lmax is the maximum length of the connection, Tg1 and Tg2 are guard times between the bursts and T1 is the length of the burst (N bits). As the figure shows, a transceiver has, between the transmission and reception phases, a specific, mandatory waiting time. The longer it takes for data to cross the connection, the longer the waiting time.
A significant advantage in the principle described above is that the transceiver part can be implemented by using one transceiver component, for example, in the case of the optic transmission connection, by using an optic component which operates, alternatively, in transmission and reception states. This kind of a "one chip solution" offers significant savings, for example, when implementing optic subscriber connections.
The drawback of the method described above is mostly that the buffering time and delay increase as the length of the connection increases. If the length of the connection is, for example, 20 kilometers, the mere propagation time across the connection is approx. 0.1 ms, whereby a buffering capacity corresponding to a traffic amount of approximately 0.2 ms is needed. As the transmission rates increase, the buffers become longer and longer, whereby the implementation becomes more and more complicated.
The delay caused by buffering and by the waiting time required by such a ping-pong type protocol generates undesired effects as it cumulates with other delays generated in the network, e.g. echo effects in voice transfer. Furthermore, certain applications are such that they hardly tolerate any delay at all, e.g. remotely controlled applications in which, for example, different processes are controlled via the data transmission network. On a general level it can be stated that at least for real-time services it is always desirable to minimize the delay, as excessive delay makes the implementation of such services impossible.